The disclosure that a convicted paedophile molested up to 60 pupils at a prestigious school has shown the need for greater monitoring of teachers who work abroad, its chairman of governors has said.

Sir Chris Woodhead also warned that the case of former teacher William Vahey, who had worked at 10 schools in nine countries, highlighted his belief "devious and sophisticated" offenders could "slip through the net" even where there were the strictest safeguards in place.

An inspection report in 2010 had warned of a weakness in Southbank International School's staff vetting measures, however Sir Chris said it was "irrelevant" in Vahey's case as his background checks had no record of his offending history.

Sir Chris said there had never been a single complaint against Vahey, who taught history and geography at the London-based school from 2009 to 2013, but did detail an incident involving one pupil, where a school investigation subsequently found "nothing untoward" had occurred, Sir Chris added.

American national Vahey was found dead last month after US police filed a warrant to examine a computer drive belonging to him, later found to contain images of at least 90 abused boys aged 12 to 14, and thought to have been taken during school trips since 2008.

The former head of Ofsted Sir Chris spoke of his "horror" on being told by the US authorities on Tuesday that images of between 50 and 60 Southbank students were included on that drive.

He was also informed the 64-year-old had - unknown to his employers - been convicted for child molestation in California in 1969 while working as a swimming instructor, but had failed to sign the sex offenders register in 1970 and simply "slipped off the radar".

Vahey later qualified as a teacher in the US in 1986, and went on to work in schools across the globe including the Middle East, before being hired at Southbank, latterly moving to teach in Nicaragua, Sir Chris added.

Sir Chris, who took up his role on the governing board a year after Vahey was hired, said there had been "one incident" involving the teacher and a pupil at the school during a trip.

"Apparently a boy was ill on a trip and Vahey took this boy into his room to look after him," he said.

"It wasn't picked up at the time but a few months later a teacher heard gossiping on a minibus and the incident was investigated.

"The boy's parents were talked to, the boy talked to, Vahey talked to, and the boy's parents agreed that there was nothing untoward and the matter shouldn't be pursued."

Sir Chris said the incident occurred while "all this was going on", and said he had asked himself if the school could have done more to safeguard pupils.

However, he said there had been "no complaints" made against Vahey and, describing the situation as "a horror", added it "beggars belief this number of children can be abused and no complaints surface, so I ask myself the question, what can we do?"

Southbank school's vetting procedures were called in to question in a 2010 report by the Independent Schools Inspectorate, a year after Vahey was hired.

However, Sir Chris said while true an "administrative weakness" was highlighted by inspectors in the school's recording of criminal records bureau (CRB) checks regime, it was "irrelevant" to what has since emerged because Vahey's CRB file had never been marked with his 1969 conviction.

"I think it is malicious and irrelevant to suggest this (the inspection report) cast any negative light on the Bill Vahey incident because as point of fact Vahey's CRB was fine," he said.

Parents of current pupils have been written to and urged to come forward with any concerns, while the families of past students are in the process of being contacted by police with the assistance of the school according to Sir Chris.

The FBI, which is leading the investigation and being assisted by the Metropolitan Police, said the photos were catalogued with dates and locations that corresponded with overnight trips that Vahey had taken with students since 2008.

The agency also believes Vahey abused children at other schools around the world where he worked.

The teacher's victims appeared to have been drugged and unconscious on the images discovered, according to investigators, meaning victims may not know they were abused.

The US authorities said Vahey had apparently killed himself two days after he was filed with the warrant - he was found dead in Luverne, Minnesota on March 21.

Vahey was first confronted about the images by a colleague at the American Nicaraguan School in Managua, Nicaragua, where he had most recently been teaching, and he confessed that he was molested as a child and had preyed on boys all his life, plying them with sleeping pills before abusing them.

The FBI's special agent Patrick Fransen said: "I've never seen another case where an individual may have molested this many children over such a long period of time.

"I'm concerned that he may have preyed on many other students prior to 2008."

Calling for tighter international controls, Sir Chris said the case "raises questions" about why Vahey was never pursued by the US authorities for failing to sign the sex offenders register and about how information was shared on teachers who work in different schools abroad.

"How did he qualify as a teacher in the United States, how is it this information was never available to any of the schools across the world who employed him over the next 40 years?

"We have to ask, is there any way schools can reasonably gather more information than they currently do?

"I don't have answers to these questions.

"I'm sure the Department for Education (DfE) and UK authorities are going to be asking themselves that."

He added: "Ultimately, there are always going to be people who slip through the net therefore the issue is how can schools as communities develop the culture where there is trust and confidence so any suspicions, any doubts, can be investigated."

The DfE has said it is monitoring the on-going investigation.

Peter Watt, of the NSPCC, said: "This is a disturbing case which is obviously causing a lot of anxiety for parents of children who went to the school. So we would urge anyone who has any concerns to contact us on 0808 800 5000 or speak to the police.

"We have trained counsellors on standby round the clock to give advice and support at what is an extremely difficult time."